With the increasing demands of mobile devices such as mobile phones, non-volatile memories and associated technology are being rapidly researched by many developers throughout the world. A representative example of the non-volatile memory is a flash memory. The flash memory is operated at low speed, such that it has been widely used as a programmable ROM.
In the meantime, a high-speed DRAM is required for an operation memory. A memory for portable or mobile devices includes not only the flash memory but also the DRAM. If a semiconductor device including characteristics of the above-mentioned two memories (i.e., the flash memory and the DRAM) can be implemented, the flash memory and the DRAM can be integrated into a single chip, and this integrated chip can substitute for all the semiconductor memories, resulting in the increasing advantages of the integrated chip of the flash memory and the DRAM.
A representative memory based on a phase change (i.e., a phase-change memory) has been disclosed in 2001 EDM (International Electron Device Meeting) by Intel Corporation.
The principles of the operations of the above-mentioned phase-change memory will be described in detail. The phase-change memory uses a material called “chalcogenide” as a memory node. The chalcogenide has different resistances according to crystal states. The chalcogenide is a material used for a recording medium such as DVD or CD. Representative examples of the chalcogenide are a Ge—Sb—Te-based material and an Ag—In—Sb—Te-based material, which include at least antimony (Sb) and tellurium (Te). A basic memory cell includes a selection transistor and the chalcogenide. In other words, the basic memory is similar to the DRAM cell, and may be considered to be a specific memory capable of replacing a capacitor with the chalcogenide. The crystal of the chalcogenide is configured in the form of a single crystal or amorphous material. The chalcogenide has different resistances changing from 10 times to 10,000 times. The chalcogenide is configured in the form of a solid memory using the difference in resistances. For example, since a magnetic RAM (MRAM) acting as anon-volatile memory has a resistance-change rate of about 40%, the phase-change memory has a greater resistance-change rate higher than that of the MRAM, such that the phase-change memory can more easily detect data than the MRAM.
In order to change a crystal state of the chalcogenide to another state, Joule heat created by the voltage is used. In order to change the chalcogenide to the amorphous material, the chalcogenide is heated at about 600° C., such that the chalcogenide is melted and is rapidly cooled. In order to crystallize the chalcogenide, it is maintained at a temperature of about 400° C. for a predetermined time of about 50 nsec. Therefore, the pulse shown in FIG. 2 is assigned to the data recording. In the case of reading the data, a word line is switched on, and two-value information (“0” and “1”) is identified by a current signal flowing between a common ground line and a bit line.
A representative phase-change memory capable of changing a crystal state of the chalcogenide to another state by a current signal received from a diode has been disclosed in U.S. Pat. Nos. 5,166,758 and 5,536,947. Another representative phase-change memory for connecting a transistor to the chalcogenide in the form of a cascade, allowing a current signal to be generated from the transistor, and changing a crystal state of the chalcogenide to another state has been disclosed in “2002 ISSCC, 12.4 Ovonic Unified Memory”, p 202 on 2002, proposed by M. Grill.